The Tropospheric Seasonally Varying Mean Climate over the Western
Hemisphere (1979-1995)

4. Northern Hemisphere seasonal variations

Large seasonal variations in SLP are evident over the middle
and high latitudes of the North Pacific and North Atlantic Oceans
(e.g., compare Figs. 5, 12, 17 and 19). Strong cyclonic flow and
low pressure dominate the regions of the Aleutian Islands and
Iceland during the boreal winter (Fig. 17), while during the boreal
summer (Fig. 19) high pressure and anticyclonic flow prevail over
the middle latitudes of both oceans. There is a marked northwestward
shift from winter to summer in the central positions, and an increase
in central pressures, of the North Pacific and North Atlantic
subtropical high pressure systems. The opposite trend in pressure
is evident over the US, with SLP being higher during winter and
lower during summer.

The westerlies over the central North Pacific and eastern North
America weaken and the maxima (jet streams) shift northward from
winter to summer (Figs.21, 27, 33 and 35). The northward shift
of the westerlies over eastern North America is most pronounced,
with the maximum westerlies shifting from the Southeast US during
January-March to southern Canada during July and August. The upper-tropospheric
circulation over North America switches from cyclonic during winter
to anticyclonic during summer, associated with the seasonal variation
in thermal forcing over the continent.

The strong jet exit region over the eastern North Pacific during
boreal winter (Fig. 33) is accompanied by a pattern of ascending
(descending) motion in the left (right) front quadrant of the
jet. Precipitation is heaviest over the eastern Pacific in the
poleward portion of the jet exit region (Fig. 17). Orographic
effects enhance upward vertical motion and precipitation along
the west coast of North America from British Columbia south through
the northwestern US. The maximum in precipitation over the eastern
North Pacific is weaker during summer (Fig. 19) and does not extend
as far eastward. This is consistent with the increase in SLP and
northwestward shift of the subtropical high over the eastern North
Pacific, and the weaker jet exit region during summer (compare
Figs. 17, 19, 33, and 35).

Weak low-level southerly flow over the southern US during winter
(Fig. 81) is replaced by substantially stronger southerly flow
during the spring, summer and fall seasons (Figs. 82, 83 and 84)
. Precipitable water increases over the southern US during spring,
reaches a maximum during summer and decreases in the fall. The
annual cycle of precipitable water in this region partly reflects
the annual cycle in the low-level advection of moisture from the
Gulf of Mexico northward over the region, and partly reflects
the increase in temperature and moisture carrying capacity of
the air during the warm season. Over the north-central US and
western High Plains precipitation is greatest during summer in
association with the higher amounts of precipitable water and
lower static stability.